Refrigeration system with multiple compressors and a common oil equalization line, equipped with an oil pump device.

Abstract

The refrigeration system with multiple compressors, comprising a control unit and a device (7) with multiple compressors, the device comprising at least three compressors (8) coupled in parallel, each compressor (8) being equipped with an oil sump and an oil level detection device (15), the compressors (8) being divided into two groups, each comprising at least one compressor (8); the suction ports of all compressors (8) being fluidically connected by a common suction line (16) and the oil sumps of all compressors (8) being fluidically connected by a common oil equalization line (23).The device (7) with multiple compressors includes an oil pump device (27) located in the common oil equalization line (23) and between the two groups, and at least one valve (29) located in the common oil equalization line (23) and configured to control an oil flow between the oil pump device (27) and at least one compressor (8) of a group associated with the at least one valve (29).

Description

Field of the invention

[0001] The present invention relates to a refrigeration system with multiple compressors and a method for balancing oil in a refrigeration system with multiple compressors. State of the art

[0002] Proper oil balancing is an important issue in multi-compressor systems.

[0003] The most common solutions include the provision of oil equalization lines between compressors, oil level detection devices in each compressor, and pumps to provide oil flow in the oil equalization lines between compressors, as well as a controller to control the operation of the multi-compressor system. The general idea is that once the oil level detection detects a low oil level in a particular compressor, the controller can activate the respective oil pump located in the respective oil equalization line and pump some oil from the compressor with a high oil level to the compressor with a low oil level.

[0004] While such a simple solution is sufficient for multi-compressor systems with a relatively small number of compressors (such as two or three), it becomes very difficult and, in particular, expensive to implement such a solution in multi-compressor systems with higher numbers of compressors. In particular, the price of the oil pump contributes to the final cost of a multi-compressor system.

[0005] Achieving a good oil balance in a tandem configuration is generally not that difficult and can, in most cases, be achieved by adding passive features (suction disc, throttle, etc.) to the suction line. This has only a limited impact on the pressure drop and efficiency of the overall refrigeration system. In such cases, there is little need for external actuators such as oil pumps.

[0006] However, when it comes to a larger refrigeration system with multiple compressors, the situation can change drastically. In triple configurations with passive devices embedded in the suction line, the refrigeration system experiences such a pressure drop, which negatively impacts efficiency, that the use of external actuators, such as oil pumps, becomes very useful.

[0007] DE1915174 A1 discloses an oil level compensation concept, where each compressor in a multi-compressor system has an oil level sensor and an oil pump. Based on the output from the oil level sensors, a controller activates an oil flow from the compressor with excess oil to the compressor with a low oil level. However, such a solution is complex and expensive, as it requires as many pumps as compressors.

[0008] EP0840870 A1 discloses an oil level control device for controlling the oil level in each compressor of a multi-compressor system. Specifically, each compressor is fluidly connected to an additional oil reservoir, and the oil level control device is capable of maintaining the oil level within a compressor between a predetermined minimum level and a predetermined maximum level. If the oil level in a compressor falls below the predetermined minimum level, the compressor is refilled with oil from the additional oil reservoir. If the oil level in a compressor exceeds the predetermined maximum level, oil is returned from the compressor to the additional oil reservoir. However, such a solution is also complex and expensive, as additional oil reservoirs and piping are required.

[0009] CN100516682C discloses the idea of ​​a system with multiple compressors being divided into groups, each containing an oil pump, with each group comprising at least two compressors. Such an arrangement allows for oil balancing between groups. However, it does not solve the problem when only one specific compressor in the group requires oil. Therefore, such an arrangement can lead to the generation of excessive oil levels in compressors.

[0010] JPH04116349A discloses a system with multiple compressors and an oil pump that can switch its flow direction. However, in this arrangement, the oil pump is located at a considerable distance from the compressors, which impairs efficiency. Furthermore, this solution does not solve the problem of a large number of compressors in a multi-compressor system. Furthermore, an additional oil tank is required because direct oil flow between the compressors is not possible. Summary of the invention

[0011] An object of the present invention is to provide an improved multi-compressor refrigeration system that can overcome the disadvantages encountered in conventional multi-compressor refrigeration systems.

[0012] In particular, it is an object of the present invention to provide a multi-compressor refrigeration system that is simple, reliable and cost-effective, while ensuring reliable oil balancing even when the multi-compressor system includes a large number of compressors.

[0013] According to the invention, such a multi-compressor refrigeration system comprises a controller configured to control the operation of the multi-compressor refrigeration system and a multi-compressor device comprising: - at least three compressors coupled in parallel, each compressor being provided with a suction port, an oil sump, and an oil level detection device operatively connected to the controller and configured to detect an oil level in the respective oil sump, wherein the compressors in the multi-compressor device are divided into at least two groups, each group containing at least one compressor and at least one group containing at least two compressors, - a common suction line configured to fluidly connect the suction fittings of all compressors of the multi-compressor device; - a common oil equalization line configured to fluidly connect the oil sumps of all compressors of the multi-compressor device; - an oil pumping device located in the common oil equalization line and operatively connected to the control, the oil pumping device comprising at least one oil pump;characterized in that the oil pumping device is located between two groups, such that one of the two groups is located on a first side of the oil pumping device and the other of the two groups is located on a second side of the oil pumping device, and in that the multi-compressor device further comprises at least one valve located in the common oil equalization line and associated with a group containing at least two compressors, wherein the at least one valve is operatively connected to the controller and configured to control oil flow in the common oil equalization line and between the oil pumping device and at least one compressor of the group associated with the at least one valve, wherein the controller is configured to control operation of the oil pumping device and the at least one valve based on outputs from the oil level detection devices;

[0014] Due to such an arrangement of the oil pumping device and the at least one valve, it is possible to obtain a simple and cost-effective multi-compressor system with a very limited amount of additional equipment. The present invention makes it possible to achieve reliable oil balancing without disturbing the common suction line. Therefore, the common suction line can be designed to be fully optimized for efficiency, thus improving the efficiency of the multi-compressor refrigeration system. Furthermore, the cost of a valve is much lower than that of an oil pump, making it advantageous to replace oil pumps with valves.

[0015] The multi-compressor refrigeration system may also include one or more of the following features, either alone or in combination.

[0016] According to one embodiment of the invention, the compressors in the multi-compressor device are divided into two groups.

[0017] According to one embodiment of the invention, each compressor has an oil balancing connection, wherein the common oil balancing line is configured to fluidically connect the oil balancing connections of all compressors of the device with multiple compressors.

[0018] According to one embodiment of the invention, the controller is configured to determine a current amount of oil in each compressor based on outputs from the oil level detection devices.

[0019] According to one embodiment of the invention, the controller is configured to: - to monitor the oil level in the oil sump of each compressor by means of the respective oil level detection device; - for example, based on a signal output by an oil level detection device, to detect that a low oil level situation, also called oil starvation situation, occurs in a respective compressor when the oil level in the compressor reaches a predetermined low oil level state, and - to carry out an oil balancing process when a low oil level situation is detected in a compressor, wherein the oil balancing process includes controlling the operation of the oil pumping device and the at least one valve to pump oil from the oil sump of at least one compressor of the multi-compressor device in which no low oil level situation was detected to the oil sump of the compressor in which a low oil level situation was detected.

[0020] According to one embodiment of the invention, each oil level detection device is configured to output a low oil level warning signal when the oil level in the oil sump of the respective compressor reaches a predetermined low oil level value.

[0021] According to one embodiment of the invention, the predetermined oil level condition is reached for a compressor when a low oil level warning signal is output by the respective oil level detection device.

[0022] According to one embodiment of the invention, the predetermined low oil level condition for a compressor is reached when: - the low oil level warning signal output by the respective oil level detection device is output continuously for a first predetermined time, for example for at least ten seconds, or - the low oil level warning signal output by the respective oil level detection device has switched, i.e. turned on and off, more than a predetermined number of times, and for example more than ten times within a second predetermined time and for example during the last minute.

[0023] According to one embodiment of the invention, the controller is configured to detect that an excess oil situation occurs in a respective compressor when the oil level in the compressor reaches a predetermined high oil level condition.

[0024] According to one embodiment of the invention, the oil balancing process includes controlling the operation of the oil pumping device and the at least one valve to pump oil from the oil sump of at least one compressor of the multi-compressor device in which an excess oil situation has been detected to the oil sump of the compressor in which a low oil level situation has been detected.

[0025] According to one embodiment of the invention, the oil balancing process includes: - if oil balancing is required between compressors from different groups, controlling the operation of the oil pumping device and the at least one valve to pump oil from the oil sump of at least one compressor in which no low oil level situation has been detected and which belongs to a group different from the group containing the compressor in which a low oil level situation has been detected, to the oil sump of the compressor in which a low oil level situation has been detected, - when oil balancing is required between compressors from the same group, controlling the operation of the oil pumping device and the at least one valve to pump oil from the oil sump of at least one compressor in which no low oil level situation has been detected and which belongs to the same group as the compressor in which a low oil level situation has been detected, to a temporary oil reservoir and then from the temporary oil reservoir to the oil sump of the compressor in which a low oil level situation has been detected, wherein the temporary oil reservoir is formed by the oil sump of at least one compressor belonging to a group different from the group containing the compressor in which a low oil level situation has been detected.

[0026] According to one embodiment of the invention, the controller is configured to determine the amount of oil that must be delivered to a compressor in which a low oil level situation has been detected in order to reach a predetermined oil level in the compressor.

[0027] According to one embodiment of the invention, the controller is configured to calculate a risk of occurrence of a low oil level in a compressor under a given operating condition and to control the operation of the oil pumping device and the at least one valve based on the calculation.

[0028] According to one embodiment of the invention, the controller is configured to control the oil pumping device and the at least one valve to deliver more oil to a compressor where there is a high risk of a low oil level occurring and / or to control the oil pumping device and the at least one valve not to remove oil from a compressor where there is a high risk of a low oil level occurring.

[0029] According to one embodiment of the invention, the at least one oil pump includes a volumetric oil pump configured to measure and dispense an amount of pumped oil.

[0030] According to one embodiment of the invention, the oil pump is a variable speed oil pump.

[0031] According to one embodiment of the invention, the at least one oil pump comprises two disposable oil pumps, for example two volumetric disposable oil pumps, which are connected in parallel.

[0032] According to one embodiment of the invention, the at least one oil pump includes a bidirectional oil pump and, for example, a bidirectional volumetric oil pump.

[0033] According to one embodiment of the invention, the oil pumping device comprises only one bidirectional oil pump. Such a configuration of a multi-compressor refrigeration system facilitates the control of the multi-compressor refrigeration system and significantly reduces its manufacturing costs.

[0034] According to one embodiment of the invention, the oil pump is an electromagnetic oil pump, for example, a bidirectional electromagnetic volumetric oil pump. Advantageously, the electromagnetic oil pump comprises a motor, a pumping element, and an electromagnetic coupling arranged between the pumping element and the motor. Such an oil pump ensures static sealing and prevents the risk of leaks. Furthermore, the multi-compressor refrigeration system with such an oil pump can obtain appropriate certifications for use in highly hazardous conditions, such as potentially explosive atmospheres.

[0035] According to one embodiment of the invention, the at least one valve comprises a normally closed electromagnetic valve.

[0036] According to one embodiment of the invention, the common suction line includes a main suction line and suction branches, each connecting the main suction line to the suction connection piece of a respective compressor.

[0037] According to one embodiment of the invention, the common oil equalization line includes a main oil equalization line and oil equalization branches, each connecting the main oil equalization line to the oil sump of a respective compressor, and in particular to an oil equalization connection provided on a compressor housing of a respective compressor and fluidically connected to the respective oil sump. In particular, each oil equalization branch is connected to a main oil equalization line at an oil equalization line branch.

[0038] According to one embodiment of the invention, the at least one valve includes a three-way valve located at an oil equalization line branch of the common oil equalization line and configured to control oil flow between the oil pumping device and two respective oil equalization branches of the common oil equalization line, which are fluidly connected to each other at the oil equalization line branch. In other words, the three-way valve can control oil flow between the oil pumping device and two compressors, each of which is fluidly connected to oil equalization branches, which are fluidly connected to the oil equalization line branch. Thus, the three-way valve can control oil flow to more than one compressor.

[0039] According to one embodiment of the invention, the at least one valve includes a two-way valve located in an oil equalization branch of the common oil equalization line and fluidically connected to a respective compressor, and configured to control an oil flow between the oil pumping device and the respective compressor fluidically connected to the oil equalization branch. In other words, this two-way valve can control an oil flow to only one compressor.

[0040] According to one embodiment of the invention, each group includes at least two compressors.

[0041] According to the embodiment of the invention, the multi-compressor device includes a plurality of two-way valves, each located in a respective oil equalization branch of the common oil equalization line, wherein each two-way valve is operatively connected to the controller and configured to control an oil flow between the oil pumping device and a respective compressor fluidly connected to the oil equalization branch, wherein the controller is configured to control the operation of the oil pumping device and the two-way valves based on outputs from the oil level detection devices.

[0042] According to the embodiment of the invention, each oil equalization branch is provided with a respective two-way valve.

[0043] According to one embodiment of the invention, at least one compressor of the multi-compressor device comprises at least two oil level detection devices.

[0044] According to one embodiment of the invention, the oil level detection device of each compressor includes an oil level sensor or an oil level switch.

[0045] According to one embodiment of the invention, each compressor of the multi-compressor device is a scroll compressor.

[0046] According to an embodiment of the invention, each oil level detection device is located at least partially in the respective oil sump.

[0047] According to one embodiment of the invention, each compressor includes a compressor housing and the respective oil sump is arranged in a lower part of the compressor housing.

[0048] According to one embodiment of the invention, the multi-compressor refrigeration system further comprises a refrigerant circuit including, in sequence, a condenser, an electronic expansion valve, an evaporator, and the multi-compressor device connected in series.

[0049] According to one embodiment of the invention, at least one compressor of the multi-compressor device is a variable speed compressor.

[0050] According to one embodiment of the invention, at least one compressor of the multi-compressor device is a fixed speed compressor.

[0051] According to other embodiments of the invention, the compressors may be of a type other than scroll compressors, ie the compressors may in particular be screw compressors or piston compressors.

[0052] According to one embodiment of the invention, the multi-compressor refrigeration system uses a low GWP (Global Warming Potential) refrigerant.

[0053] The present invention also relates to a method for balancing oil in a multi-compressor refrigeration system, comprising a controller configured to control the operation of the multi-compressor refrigeration system, and a multi-compressor device including at least three compressors coupled in parallel, each compressor being equipped with a suction port, an oil sump, and an oil level detection device operatively connected to the controller and configured to detect an oil level in the respective oil sump, the oil sumps of all compressors of the multi-compressor device being fluidly connected by a common oil equalization line, and the suction ports of all compressors of the multi-compressor device being fluidly connected by a common suction line, the method comprising: - dividing the compressors of the multi-compressor device into at least two groups, each group containing at least one compressor and at least one group comprising at least two compressors, - Providing an oil pumping device in the common oil equalization line and between two groups, such that one of the two groups is located on a first side of the oil pumping device and the other of the two groups is located on a second side of the oil pumping device, wherein the oil pumping device comprises at least one oil pump, - Providing at least one valve located in the common oil equalization line, wherein the at least one valve is assigned to a group containing at least two compressors and is configured to control an oil flow in the common oil equalization line and between the oil pumping device and at least one compressor of the group assigned to the at least one valve, - Monitoring an oil level in the oil sump of each compressor by the respective oil level detection device, - Detecting that a low oil level situation occurs in a compressor when the oil level in that compressor reaches a predetermined low oil level condition, - performing an oil balancing operation when a low oil level situation is detected in a compressor, wherein the oil balancing operation includes controlling the operation of the oil pumping device and the at least one valve to pump oil from the oil sump of at least one compressor of the multi-compressor device in which no low oil level situation was detected to the oil sump of the compressor in which a low oil level situation was detected.

[0054] Thanks to the presence of at least one valve, it is possible to precisely remove excess oil from a selected compressor and then pump the oil to a selected compressor with a low oil level. This makes it possible to maintain the oil level in each compressor at the desired level, which is usually above the minimum oil level and below the maximum oil level.

[0055] According to one embodiment of the invention, the oil balancing process includes: - if oil balancing is required between compressors from different groups, conveying, by controlling the operation of the oil pumping device and the at least one valve, oil from the oil sump of at least one compressor in which no low oil level situation has been detected and which belongs to a group different from the group containing the compressor in which a low oil level situation has been detected, to the oil sump of the compressor in which a low oil level situation has been detected, - when oil balancing is required between compressors from the same group, conveying, by controlling the operation of the oil pumping device and the at least one valve, oil from the oil sump of at least one compressor in which no low oil level situation has been detected and which belongs to the same group as the compressor in which a low oil level situation has been detected, to a temporary oil reservoir and then from the temporary oil reservoir to the oil sump of the compressor in which a low oil level situation has been detected, wherein the temporary oil reservoir is formed by the oil sump of at least one compressor belonging to a group different from the group containing the compressor in which a low oil level situation has been detected.

[0056] This makes it possible to balance oil within a refrigeration system with multiple compressors without the need for additional external oil tanks. If a compressor with a low oil level and a compressor with an excess oil level are located in different groups, the oil pumping device located between the groups simply pumps oil from the group with the excess oil to the group with the oil deficiency. If a compressor with a low oil level and a compressor with an excess oil level are located in the same group, the oil pump first pumps the excess oil from that group to the other group (the other group acts as a temporary oil tank) and then back to the group with the compressor with the low oil level.

[0057] According to one embodiment of the invention, the method includes dividing a refrigeration system with multiple compressors into two groups.

[0058] According to one embodiment of the invention, the method includes calculating a risk of a low oil level occurring in a compressor under a given operating condition and controlling the operation of the oil pumping device and the at least one valve based on this calculation, for example, to supply more oil to a compressor with a high risk of a low oil level occurring and / or controlling the operation of the oil pumping device and the valves so as not to withdraw oil from a compressor with a high risk of a low oil level occurring. Advantageously, the method includes selecting at least one oil sump from which oil is pumped for oil production based on the calculation.

[0059] According to one embodiment of the invention, the method includes outputting a low oil level warning signal from an oil level detection device and to the controller when the oil level in the oil sump of the respective compressor reaches a predetermined low oil level value. Short description of the drawings

[0060] The following detailed description of two embodiments of the invention will be better understood when read in conjunction with the attached drawings, it being understood, however, that the invention is not limited to the specific embodiments disclosed. Fig. 1 is a schematic diagram of a multi-compressor refrigeration system according to a first embodiment of the invention. Fig. 2 is a longitudinal sectional view of a compressor of the multi-compressor refrigeration system of Fig. 1. Fig. Figure 3 is a schematic representation of a multi-compressor device of the multi-compressor refrigeration system of Fig. 1. Fig. Figure 4 is a diagram of a method for balancing oil in the multi-compressor refrigeration system of Fig. 1. Fig. 5 is a schematic diagram of a multi-compressor apparatus of a multi-compressor refrigeration system according to a second embodiment of the invention. Fig. 6 is a schematic diagram of a multi-compressor apparatus of a multi-compressor refrigeration system according to a third embodiment of the invention. Fig. 7 is a schematic diagram of a multi-compressor apparatus of a multi-compressor refrigeration system according to a fourth embodiment of the invention. Fig. 8 is a schematic diagram of a multi-compressor apparatus of a multi-compressor refrigeration system according to a fifth embodiment of the invention. Detailed description of the invention

[0061] The Fig. 1 to 3 describe a refrigeration system 2 with several compressors, comprising a refrigerant circuit 3 which successively includes a condenser 4, an electronic expansion valve 5, an evaporator 6 and a device 7 with several compressors connected in series.

[0062] The multi-compressor device 7 comprises at least three compressors 8 coupled in parallel. According to the Fig. In the embodiment shown in Figure 1, the multi-compressor device 7 comprises three compressors 8. However, the multi-compressor device 7 may comprise more than three compressors 8 and, for example, four or five compressors 8. Each compressor 8 may, for example, be a variable-speed compressor.

[0063] Each compressor 8 includes a compressor housing 9 provided with a suction port 11 configured to supply the respective compressor 8 with refrigerant gas to be compressed and an outlet port 12 configured to discharge compressed refrigerant gas. The multi-compressor refrigeration system 2 can use a low GWP (Global Warming Potential) refrigerant as the refrigerant gas.

[0064] Advantageously, each compressor 8 is a scroll compressor and includes a compression unit 13 arranged within the respective compressor housing 9 and configured to compress the refrigerant gas supplied from the respective suction port 11. Each compression unit 13 includes a fixed scroll fixed with respect to the respective compressor housing 9 and an orbiting scroll configured to perform an orbiting movement with respect to the respective fixed scroll during operation of the respective compressor 8. However, according to another embodiment of the invention, the compressors 8 may be of a type other than scroll compressors and may, in particular, be screw compressors or piston compressors.

[0065] Each compressor 8 also includes an oil sump 14 arranged at a lower part of the respective compressor housing 9, and an oil level detection device 15 located in the respective oil sump 14 and configured to detect an oil level in the respective oil sump 14. Each oil level detection device 15 can be an oil level sensor or an oil level switch. Advantageously, each oil level detection device 15 is configured to output a low oil level warning signal when the oil level in the oil sump 14 of the respective compressor 8 reaches a predetermined low oil level value.

[0066] The device 7 with multiple compressors further includes a common suction line 16, which is configured to fluidically connect the suction connection piece 11 of all compressors 8 of the device 7 with multiple compressors. The common suction line 16 includes, in particular, a main suction line 17 and suction branches 18, each connecting the main suction line 17 to the suction connection piece 11 of a respective compressor 8. The device 7 with multiple compressors also includes a common outlet line 19, which is configured to fluidically connect the outlet connection pieces 12 of all compressors 8 of the device 7 with multiple compressors. The common outlet line 19 includes, in particular, a main outlet line 21 and outlet branches 22, each connecting the main outlet line 21 to the outlet connection piece 12 of a respective compressor.

[0067] Furthermore, the device 7 with multiple compressors includes a common oil equalization line 23 configured to fluidically connect the oil sumps 14 of all compressors 8 of the device 7 with multiple compressors. The common oil equalization line 23 includes, in particular, a main oil equalization line 24 and oil equalization branches 25, each connecting the main oil equalization line 24 to the oil sump 14 of a respective compressor 8 and, in particular, to an oil equalization connection 26 provided on the compressor housing 9 of a respective compressor 8 and fluidically connected to the respective oil sump 14. In particular, each oil equalization branch 25 is connected to the main oil equalization line 24 at an oil equalization line branch 26.

[0068] The compressors in the multi-compressor device 7 are divided into two groups, each group containing at least one compressor 8 and at least one group containing at least two compressors 8. According to the Fig. 3, one group (referred to as the first group) comprises one compressor 8, and the other group (referred to as the second group) comprises two compressors 8.

[0069] The device 7 with multiple compressors further includes an oil pumping device 27, which is located in the common oil equalization line 23 and between the two groups, so that one of the two groups is located on a first side of the oil pumping device 27 and the other of the two groups is located on a second side of the oil pumping device 27. According to the Fig. In the embodiment shown in Figure 3, the oil pumping device 27 comprises an oil pump 28, such as a bidirectional volumetric oil pump and, for example, a bidirectional electromagnetic volumetric oil pump. Advantageously, the oil pump 28 is configured to measure and dispense a quantity of pumped oil.

[0070] According to the Fig. In the embodiment shown in Figure 3, the device 7 with multiple compressors further includes two valves 29 located in the common oil equalization line 23 and assigned to the group containing two compressors 8. Advantageously, each valve 29 is a normally closed electromagnetic two-way valve and is located in an oil equalization branch 25 of the common oil equalization line 23 and thus downstream of the respective compressor 8. Each valve 29 is thus configured to control an oil flow between the oil pumping device 27 and the compressor 8 fluidly connected to the respective oil equalization branch 25. In other words, each valve 29 is configured to control the fluidic connection between the oil pumping device 27 and the respective compressor 8.

[0071] The multi-compressor refrigeration system 2 further includes a controller 31 configured to control the operation of the multi-compressor refrigeration system 2, and in particular to control the operation (starting or stopping) of the compressors 8. The controller 31 may include, for example, a microprocessor and a memory.

[0072] The controller 31 is operatively connected to each oil level detection device 15, to the oil pumping device 27, and to each valve 29, and is specifically configured to control the operation of the oil pumping device 27 and the valves 29 based on the outputs of the oil level detection devices 15.

[0073] Fig. Figure 4 shows a diagram of a method for balancing oil in the refrigeration system 2 with multiple compressors of Fig. 1.

[0074] The procedure includes in particular: - monitoring an oil level in the oil sump 14 of each compressor 8 by the respective oil level detection device 15; - detecting, by the controller 31 and based on a signal output by an oil level detection device 15, that a low oil level situation, also referred to as an oil shortage situation, occurs in a corresponding compressor 8 when the oil level in the compressor 8 reaches a predetermined low oil level state, and - Performing an oil balancing procedure when a low oil level situation is detected in a compressor 8, the oil balancing procedure comprising the following: o if an oil balancing between compressors from different groups is required, controlling the operation of the oil pumping device 27 and the valves 29 to pump oil from the oil sump 14 of at least one compressor 8, in which no low oil level situation has been detected and which belongs to a group different from the group containing the compressor 8 in which a low oil level situation has been detected, to the oil sump 14 of the compressor 8 in which a low oil level situation has been detected, o when oil balancing is required between the compressors 8 of the same group, controlling the operation of the oil pumping device 27 and the valves 29 to pump oil from the oil sump 14 of at least one compressor 8 in which no low oil level situation has been detected and which belongs to the same group as the compressor 8 in which a low oil level situation has been detected, to a temporary oil reservoir and then from the temporary oil reservoir to the oil sump 14 of the compressor 8 in which a low oil level situation has been detected, the temporary oil reservoir being formed by the oil sump 14 of at least one compressor 8 belonging to a group different from the group containing the compressor 8 in which a low oil level situation has been detected.

[0075] Advantageously, the controller 31 is configured to determine, based on outputs from the oil level detection devices 15, a current oil quantity in each compressor 8 and the amount of oil that needs to be pumped to a compressor 8 for which a low oil level situation has been detected in order to reach a predetermined oil level in the compressor 8. Based on this, the controller 31 can operate the oil pumping device 27 to pump the exact amount of oil required for the compressor 8 for which a low oil level situation has been detected.

[0076] According to one embodiment of the invention, the predetermined low oil level condition is reached for a compressor 8 when a low oil level warning signal is output by the respective oil level detection device 15, ie, when the oil level in the oil sump 14 of the respective compressor 8 reaches the predetermined low oil level value.

[0077] Advantageously, the method further includes detecting, by the controller 31 and based on a signal output by an oil level detection device 15, that an excess oil situation occurs in a respective compressor 8 when the oil level in the compressor 8 reaches a predetermined high oil level condition. According to such an embodiment of the invention, the oil balancing process comprises: - when oil balancing is required between compressors 8 from different groups, controlling the operation of the oil pumping device 27 and the valves 29 to pump oil from the oil sump 14 of at least one compressor 8 in which an oil excess situation has been detected and which belongs to a different group than the group containing the compressor 8 in which a low oil level situation has been detected, to the oil sump 14 of the compressor 8 in which a low oil level situation has been detected, - when oil balancing is required between compressors 8 from the same group, controlling the operation of the oil pumping device 27 and the valves 29 to pump oil from the oil sump 14 of at least one compressor 8 in which an oil excess situation has been detected and which belongs to the same group as the compressor 8 in which a low oil level situation has been detected, to a temporary oil reservoir and then from the temporary oil reservoir to the oil sump 14 of the compressor 8 in which a low oil level situation has been detected, the temporary oil reservoir being formed by the oil sump 14 of at least one compressor 8 belonging to a group different from the group containing the compressor 8 in which a low oil level situation has been detected.

[0078] According to one embodiment of the invention, the predetermined high oil level condition is reached for a compressor 8 when a high oil level warning signal is output by the respective oil level detection device 15, ie, when the oil level in the oil sump 14 of the respective compressor 8 reaches a predetermined high oil level value.

[0079] According to one embodiment of the invention, the method further includes calculating a risk of a low oil level occurring in a compressor 8 at a given operating condition and controlling the operation of the oil pumping device 27 and the valves 29 based on this calculation to supply more oil to a compressor 8 at which there is a high risk of a low oil level occurring and / or controlling the operation of the oil pumping device 27 and the valves 29 not to withdraw oil from a compressor 8 at which there is a high risk of a low oil level occurring.

[0080] In the following, several scenarios of the process for balancing oil in the refrigeration system 2 with multiple compressors of Fig. 1 revealed. Scenario 1: A low oil level detected in the first group and an excessive oil level detected in the second group

[0081] In scenario 1, oil transfer is required between two different groups of compressors 8, and the oil should be transferred from the second group, which includes two compressors 8 (referred to as the second and third compressors), to the first group, which includes only one compressor 8 (referred to as the first compressor).

[0082] In such a situation, the oil balancing process involves opening the valve 29 associated with the second compressor (or the third compressor) and operating the oil pump 28 to begin pumping oil in the direction from the second group to the first group, and specifically from the second compressor (or the third compressor) in the second group to the first compressor in the first group. Once the oil level detection device 15 located in the oil sump 14 of the first compressor detects that the oil level in the oil sump 14 has reached the predetermined oil level, the oil balancing process involves stopping the operation of the oil pump 28 and closing the valve associated with the second compressor (or the third compressor).

[0083] It may happen that the amount of oil in the second compressor (or the third compressor) is insufficient for the first compressor to reach the predetermined oil level. In such a situation, the oil balancing process additionally includes operating the oil pump 28 and the valves 29 to pump oil from the third compressor (or the second compressor) to the first compressor.

[0084] It may also happen that the controller 31 determines a high risk of a low oil level occurring for the second compressor (or the third compressor). In such a situation, the oil balancing process involves controlling the operation of the oil pump 28 and the valves 29 to pump oil from the other compressor 8 with a high oil level, i.e., the third compressor (or the second compressor).

[0085] The present invention also provides for simultaneously extracting oil from more than one oil sump 14. Scenario 2: An excessive oil level detected in the first group and a low oil level detected in the second group

[0086] In this scenario, oil should be pumped from the first group to the second. Since the first group only includes one compressor 8, the oil is pumped from the first compressor to the second compressor and / or the third compressor.

[0087] In such a situation, the oil balancing process involves opening the valve 29 associated with the second compressor (and / or the third compressor) and operating the oil pump 28 to begin pumping oil in the direction from the first group to the second group, and specifically from the first compressor in the first group to the second compressor (and / or the third compressor) in the second group. Once the oil level detection device 15 located in the oil sump 14 of the second compressor (and / or the third compressor) detects that the oil level in the oil sump 14 has reached the predetermined oil level, the oil balancing process involves stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor (and / or the third compressor). Scenario 3: A low oil level and an excessive oil level detected in the same group

[0088] In this scenario, oil should be pumped within the same group. To achieve this, an intermediate step was introduced. Assume that the excessive oil level was detected in the second compressor and that the low oil level was detected in the third compressor.

[0089] First, the oil balancing process involves controlling the operation of the oil pump 28 and the valves 29 associated with the second and third compressors to pump excess oil from the second compressor to a temporary oil reservoir formed by the oil sump 14 of the first compressor. Specifically, the oil balancing process involves opening the valve 29 associated with the second compressor and operating the oil pump 28 to pump oil from the second group to the first group, and specifically from the oil sump 14 of the second compressor to the oil sump 14 of the first compressor. Once a sufficient amount of oil has been pumped to the oil sump 14 of the first compressor, the oil balancing process involves stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor.

[0090] Subsequently, the oil balancing process involves opening the valve 29 associated with the third compressor and operating the oil pump 28 to pump oil from the first group to the second group, namely from the oil sump 14 of the first compressor to the oil sump 14 of the third compressor. Once the oil level detection device 15 located in the oil sump 14 of the third compressor detects that the oil level in the oil sump 14 has reached the predetermined oil level, the oil balancing process involves stopping the operation of the oil pump 28 and closing the valve 29 of the third compressor.

[0091] The present invention also provides for the controller 31 to operate the oil pump 28 and the valves 29 to deliver oil to a compressor 8, even if no low oil level situation has been detected for the compressor 8 at that moment, because the compressor 8 is about to reach a certain operating state in which there is a high risk of a low oil level occurring. In such a situation, the method may include the controller 31 controlling the oil pump 28 and the valves 29 to deliver some additional oil to such a compressor 8 before the compressor 8 reaches this operating state.

[0092] Fig. Figure 5 discloses a multi-compressor refrigeration system 2 according to a second embodiment of the invention, which differs from the first embodiment essentially in that the multi-compressor device 7 comprises four compressors 8 and each of the first and second groups comprises two compressors 8, and in that each oil equalization branch 25 is equipped with a respective valve 29 and in particular with a normally closed electromagnetic two-way valve 29.

[0093] On one side of the oil pump 28 are the first compressor and the second compressor belonging to the first group, and on the other side of the oil pump 28 are the third compressor and the fourth compressor belonging to the second group.

[0094] The oil equalization operation for the multi-compressor refrigeration system 2 according to the second embodiment of the invention is very similar to that described above for the first embodiment of the invention and is therefore not disclosed below. In this situation, oil can be pumped to / from two compressors 8 simultaneously.

[0095] Fig. 6 discloses a refrigeration system 2 with several compressors according to a third embodiment of the invention, which differs from the second embodiment essentially in that the second group comprises three compressors 8.

[0096] The oil equalization operation for the multi-compressor refrigeration system 2 according to the third embodiment of the invention is very similar to that described above for the first embodiment of the invention and is therefore not disclosed below. In this situation, oil can be pumped to / from three compressors 8 simultaneously.

[0097] Fig. Figure 7 discloses a multi-compressor refrigeration system 2 according to a fourth embodiment of the invention, which differs from the first embodiment essentially in that the multi-compressor device 7 includes only one valve 29 (instead of two normally closed electromagnetic two-way valves), and the valve 29 is a three-way valve located at an oil equalization line branch 26 of the common oil equalization line 23. The three-way valve is specifically configured to control oil flow between the oil pumping device 27 and the two oil equalization branches 25 of the common oil equalization line 23, which are fluidly connected at the oil equalization line branch 26.

[0098] By replacing two two-way valves with one three-way valve, the refrigeration system 2 according to the invention with multiple compressors is simplified and the overall costs are reduced.

[0099] The present invention also provides for replacing two-way valves with three-way valves in the second and third embodiments of the invention.

[0100] Fig. Fig. 8 discloses a refrigeration system 2 with multiple compressors according to a fifth embodiment of the invention, which differs from the fourth embodiment essentially in that the oil pumping device 27 (instead of a bidirectional oil pump) includes two one-way oil pumps 28, for example two volumetric one-way oil pumps 28 connected in parallel.

[0101] The present invention also contemplates that such an oil pumping device 27 may replace the oil pumping devices 27 in other embodiments described above.

[0102] Based on the embodiments described above, a person skilled in the art can easily implement the present invention in a multi-compressor refrigeration system 2 comprising any number of compressors 8. Furthermore, a person skilled in the art can easily combine features described with respect to different embodiments, unless clearly stated otherwise.

[0103] Of course, the invention is not limited to the embodiments described above by way of non-limiting examples, but on the contrary includes all embodiments thereof. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 1915174 A1

[0007] EP 0840870 A1

[0008] CN 100516682C

[0009] JPH04116349A

[0010]

Claims

[1] A multi-compressor refrigeration system (2), comprising a controller (31) configured to control the operation of the multi-compressor refrigeration system (2), and a multi-compressor device (7) comprising - at least three compressors (8) coupled in parallel, each compressor (8) being provided with a suction connection piece (11), an oil sump (14) and an oil level detection device (15) operatively connected to the controller (31) and configured to detect an oil level in the respective oil sump (14), the compressors (8) in the multi-compressor device (7) being divided into at least two groups, each group containing at least one compressor (8) and at least one group containing at least two compressors (8), - a common suction line (16) configured to fluidly connect the suction connection pieces (11) of all compressors (8) of the device (7) having a plurality of compressors; - a common oil equalization line (23) configured to fluidically connect the oil sumps (14) of all compressors (8) of the device (7) with multiple compressors; - an oil pumping device (27) located in the common oil equalization line (23) and operatively connected to the control (31), wherein the oil pumping device (27) comprises at least one oil pump (28), characterized bythat the oil pumping device (27) is located between two groups, so that one of the two groups is located on a first side of the oil pumping device (27) and the other of the two groups is located on a second side of the oil pumping device (27), and that the device (7) with several compressors further comprises at least one valve (29) which is located in the common oil equalization line (23) and is assigned to a group which contains at least two compressors (8), wherein the at least one valve (29) is operatively connected to the controller (31) and is configured to control an oil flow in the common oil equalization line (23) and between the oil pumping device (27) and at least one compressor (8) of the group which is assigned to the at least one valve (29), wherein the controller (31) is configured toto control the operation of the oil pumping device (27) and the at least one valve (29) based on the outputs from the oil level detection devices (15). [2] A multi-compressor refrigeration system (2) according to claim 1, wherein the controller (31) is configured to determine a current amount of oil in each compressor (8) based on outputs from the oil level detection devices (15). [3] A refrigeration system (2) with multiple compressors according to claim 1 or 2, wherein the controller (31) is configured to: - to monitor an oil level in the oil sump (14) of each compressor (8) by the corresponding oil level detection device (15); - to detect that a low oil level situation occurs in a respective compressor (8) when the oil level in the compressor (8) reaches a predetermined low oil level condition, and - to carry out an oil balancing process when a low oil level situation is detected in a compressor (8), wherein the oil balancing process includes controlling the operation of the oil pumping device (27) and the at least one valve (29) to pump oil from the oil sump (14) of at least one compressor (8) of the device (7) with a plurality of compressors, in which no low oil level situation was detected, to the oil sump (14) of the compressor (8) in which a low oil level situation was detected. [4] A refrigeration system (2) having a plurality of compressors according to any one of claims 1 to 3, wherein the controller (31) is configured to detect that an excess oil situation occurs in a respective compressor (8) when the oil level in the compressor (8) reaches a predetermined high oil level condition. [5] A multi-compressor refrigeration system (2) according to claims 3 and 4, wherein the oil balancing process includes controlling the operation of the oil pumping device (27) and the at least one valve (29) to pump oil from the oil sump (14) of at least one compressor (8) of the multi-compressor device (7) in which an excess oil situation has been detected to the oil sump (14) of the compressor (8) in which a low oil level situation has been detected. [6] A multi-compressor refrigeration system (2) according to any one of claims 3 to 5, wherein the oil balancing process includes: - when an oil balancing between compressors (8) from different groups is required, controlling the operation of the oil pumping device (27) and the at least one valve (29) to pump oil from the oil sump (14) of at least one compressor (8) in which no low oil level situation has been detected and which belongs to a group different from the group containing the compressor (8) in which a low oil level situation has been detected, to the oil sump (14) of the compressor (8) in which a low oil level situation has been detected, - when oil balancing is required between compressors (8) from the same group, controlling the operation of the oil pumping device (27) and the at least one valve (29) to pump oil from the oil sump (14) of at least one compressor (8) in which no low oil level situation has been detected and which belongs to the same group as the compressor (8) in which a low oil level situation has been detected, to a temporary oil reservoir and then from the temporary oil reservoir to the oil sump (14) of the compressor (8) in which a low oil level situation has been detected, wherein the temporary oil reservoir is formed by the oil sump (14) of at least one compressor (8) belonging to a group different from the group containing the compressor (8) in which a low oil level situation has been detected. [7] A refrigeration system (2) with multiple compressors according to any one of claims 3 to 6, wherein the controller (31) is configured to determine the amount of oil that must be delivered to a compressor (8) in which a low oil level situation has been detected in order to reach a predetermined oil level in the compressor (8). [8] A multi-compressor refrigeration system (2) according to any one of claims 1 to 7, wherein the controller (31) is configured to calculate a risk of a low oil level occurring in a compressor (8) at a given operating condition and to control the operation of the oil pumping device (27) and the at least one valve (29) based on the calculation. [9] A refrigeration system (2) with multiple compressors according to claim 8, wherein the controller (31) is configured to control the oil pumping device (27) and the at least one valve (29) to deliver more oil to a compressor (8) at a high risk of a low oil level occurring and / or to control the oil pumping device (27) and the at least one valve (29) not to remove oil from a compressor (8) at a high risk of a low oil level occurring. [10] A multi-compressor refrigeration system (2) according to any one of claims 1 to 9, wherein the at least one oil pump (28) includes a volumetric oil pump configured to measure and dispense an amount of pumped oil. [11] A multi-compressor refrigeration system (2) according to any one of claims 1 to 10, wherein the at least one oil pump (28) includes one-way oil pumps connected in parallel. [12] A multi-compressor refrigeration system (2) according to any one of claims 1 to 10, wherein the at least one oil pump (28) comprises a bidirectional oil pump. [13] A multi-compressor refrigeration system (2) according to any one of claims 1 to 12, wherein the at least one valve (29) comprises a normally closed electromagnetic valve. [14] A multi-compressor refrigeration system (2) according to any one of claims 1 to 13, wherein the at least one valve (29) comprises a three-way valve located at an oil equalization line branch (26) of the common oil equalization line (23) and configured to control an oil flow between the oil pumping device (27) and two respective oil equalization branches (25) of the common oil equalization line (23) fluidly connected to each other at the oil equalization line branch (26). [15] A multi-compressor refrigeration system (2) according to any one of claims 1 to 14, wherein the at least one valve (29) comprises a two-way valve located in an oil equalization branch (25) of the common oil equalization line (23) and fluidically connected to a respective compressor and configured to control an oil flow between the oil pumping device (27) and the respective compressor fluidically connected to the oil equalization branch (25). [16] A multi-compressor refrigeration system (2) according to any one of claims 1 to 15, wherein at least one compressor (8) of the multi-compressor device (7) includes at least two oil level detection devices (15). [17] A method for balancing oil in a refrigeration system (2) with multiple compressors, comprising a controller (31) configured to control the operation of the refrigeration system (2) with multiple compressors, and a device (7) with multiple compressors containing at least three compressors (8) coupled in parallel, wherein each compressor (8) is equipped with a suction connection piece (11), an oil sump (14), and an oil level detection device (15) operatively connected to the controller (31) and configured to detect an oil level in the respective oil sump (14), wherein the oil sumps (14) of all compressors (8) of the device (7) with multiple compressors are fluidically connected by a common oil equalization line (23), and the suction connection pieces (11) of all compressors (8) of the device (7) with multiple compressors are fluidically connected by a common suction line (16), the method comprising the following: - dividing the compressors (8) of the device (7) with several compressors into at least two groups, each group containing at least one compressor (8) and at least one group comprising at least two compressors (8), - providing an oil pumping device (27) in the common oil equalization line (23) and between two groups, such that one of the two groups is located on a first side of the oil pumping device (27) and the other of the two groups is located on a second side of the oil pumping device, wherein the oil pumping device (27) comprises at least one oil pump (28), - Providing at least one valve (29) located in the common oil equalization line (23), wherein the at least one valve (29) is assigned to a group containing at least two compressors (8) and is configured to control an oil flow in the common oil equalization line (23) and between the oil pumping device (27) and at least one compressor (8) of the group assigned to the at least one valve (29), - monitoring the oil level in the oil sump (14) of each compressor (8) by the respective oil level detection device (15), - detecting that a low oil level situation occurs in a compressor (8) when the oil level in this compressor (8) reaches a predetermined low oil level condition, - performing an oil balancing process when a low oil level situation is detected in a compressor (8), wherein the oil balancing process includes controlling the operation of the oil pumping device (27) and the at least one valve (29) to pump oil from the oil sump (14) of at least one compressor (8) of the device (7) with a plurality of compressors, in which no low oil level situation was detected, to the oil sump (14) of the compressor (8) in which a low oil level situation was detected. [18] The method of claim 17, wherein the oil balancing process includes: - when an oil balancing between compressors (8) from different groups is required, conveying, by controlling the operation of the oil pumping device (27) and the at least one valve (29), oil from the oil sump (14) of at least one compressor (8) in which no low oil level situation has been detected and which belongs to a group different from the group containing the compressor (8) in which a low oil level situation has been detected, to the oil sump (14) of the compressor (8) in which a low oil level situation has been detected, - when oil balancing is required between compressors (8) from the same group, conveying, by controlling the operation of the oil pumping device (27) and the at least one valve (29), oil from the oil sump (14) of at least one compressor (8) in which no low oil level situation has been detected and which belongs to the same group as the compressor (8) in which a low oil level situation has been detected, to a temporary oil reservoir and then from the temporary oil reservoir to the oil sump (14) of the compressor (8) in which a low oil level situation has been detected, wherein the temporary oil reservoir is formed by the oil sump (14) of at least one compressor (8) belonging to a group different from the group containing the compressor (8) in which a low oil level situation has been detected.

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